This invention relates to the purification of gases containing elemental sulfur and optionally containing carbonaceous material.
More particularly, as will be discussed herein, this invention relates to a method for purification of sulfur dioxide containing elemental sulfur and optionally containing carbonaceous material.
One process for making sulfur dioxide is disclosed in British Pat. No. 985,784 and comprises the reaction of sulfur with sulfur trioxide in a pool of molten sulfur and highly concentrated sulfuric acid (100 to 101%). The sulfur and sulfur trioxide react below the surface of the pool at temperatures of from 120.degree. C. to 150.degree. C., producing sulfur dioxide. A similar process of this type is shown in U.S. Pat. No. 2,813,007 of Hall, et al. Another process for production of sulfur dioxide is shown in U.S. Pat. No. 2,539,579 of Hecker and comprises the reaction of gaseous sulfur trioxide with solid sulfur at temperatures below the melting point of sulfur, generally from about 50.degree. C. to about 100.degree. C. In another type of process sulfur dioxide is prepared by a gas phase reaction of sulfur and sulfur trioxide as described, for instance, in U.S. Pat. Nos. 2,156,791 of Mohr and 2,813,006 of Hayworth, et al. Still another process is that described in U.S. Pat. No. 4,053,573 of Harrer, et al., which deals with preparation of sulfur dioxide by reaction of molten sulfur with spent sulfuric acid.
In such processes, the sulfur dioxide gas produced by the reaction generally also contains elemental sulfur, as sulfur vapor and/or particles of entrained liquid or solid sulfur. In addition, if the sulfur and/or sulfuric acid utilized for the reaction contained carbonaceous impurities, such impurities may also be contained in the sulfur dioxide product gas. The presence of element sulfur (and/or carbonaceous impurities) in the sulfur dioxide gas may cause serious plugging of downstream processing and purification equipment and may result in contamination of product sulfur dioxide or other products produced from it, such as sulfuric acid.
Several techniques have been utilized or suggested in prior art operations for removing sulfur contained in the sulfur dioxide product gases. However, such techniques have either not sufficed to remove sufficient amounts of sulfur or have proven to be expensive in light of the results obtained. Alternatively such techniques may only be satisfactory for use in connection with processes which produce sulfur dioxide at rather high temperatures. For instance, British Pat. No. 985,784 discloses the scrubbing of the sulfur dioxide gas with 98% sulfuric acid. However, it has been found that such scrubbing may not be sufficient to remove all the sulfur from the product gases. U.S. Pat. No. 2,813,007 of Hall, et al. states that in order to prevent carry-over of sulfur into downstream rectification equipment an excess of sulfur trioxide must be maintained in the gas phase over the molten pool of sulfur, or sulfur trioxide must be introduced into the lower deck of the rectification section through which ascending sulfur-containing gases pass from the sulfur dioxide production reactor. In the process of U.S. Pat. No. 4,053,573 of Harrer, et al. sulfur contained in the product gases is removed by condensation. This again may not be sufficient to remove enough of the sulfur from the product gases. U.S. Pat. No. 2,813,006 of Hayworth, et al. discloses production of sulfur dioxide gas in a gas phase reaction of sulfur with sulfur trioxide. This reaction generates a product gas which contains a large amount of sulfur vapor; at column 3, line 27, it is stated that the sulfur vapor can be 17% by volume of the product gases. This sulfur vapor is removed through the by-passing of a major portion of the sulfur trioxide fed to the reactor and reacting the thus by-passed sulfur trioxide in the gas phase with the sulfur contained in the sulfur dioxide product gases, converting the sulfur to additional sulfur dioxide. Such a process requires a high temperature of at least 650.degree. F. (343.degree. C.) and is thus not suitable for use to purify the gases from a reactor in which sulfur trioxide or sulfuric acid is reacted with molten sulfur, which operates at lower temperatures. Attempts to operate such a gas-phase purification step at lower temperatures have been made and have resulted in incomplete conversion of the sulfur.
In addition to sulfur dioxide produced by such processes, sulfur contamination may also cause problems in handling and product purity of other gases containing elemental sulfur.
With one exception, none of the above-mentioned patents discloses means for removing carbonaceous substances which may be introduced into the sulfur dioxide production zone as impurities in one or another feed. The sole exception is U.S. Pat. No. 4,053,573 which removes these impurities by purging from the pool of molten sulfur. However, such impurities may also be carried downstream in the sulfur dioxide product gases. Carbonaceous impurities, in general, present the same types of problems as elemental sulfur; that is, plugging of equipment and the presence of impurities in the product or products of the plant. Sulfuric acid containing substantial amounts of carbonaceous material could not readily be disposed of as a product; it would either be recycled to the main process (if possible) or be treated as sludge.
The inability to remove carbonaceous impurities from sulfur dioxide process streams also prohibits the use of impure sulfur, e.g. "dark" sulfur as a feed to the process; only purified sulfur is acceptable.
The present invention provides a method for removal of elemental sulfur from gases, particularly gases containing primarily sulfur dioxide. It also provides an improvement in a process for production of sulfur dioxide by reaction of elemental sulfur with sulfur trioxide or sulfuric acid, in which the product sulfur dioxide gases can be purified from elemental sulfur contained therein. It further provides a method for removing carbonaceous impurities contained in gases, particularly gases containing primarily sulfur dioxide, and more particularly gases produced by the reaction of sulfur with sulfur trioxide or sulfuric acid in which carbonaceous impurities are contained in one or more of the reactants.